Intrinsic birefringence in calcium fluoride and barium fluoride

We report measurements of the intrinsic birefringence in CaF₂ and BaF₂ for wavelengths in the range 365 nm to 156 nm, and ab initio optical response calculations in these materials and Si, Ge, GaAs, and GaP. Calculations typically agree within 40%. For 〈110〉 propagation, where the effect is largest, our measurements for CaF₂[BaF₂] give ${n}_{〈1{}10〉}{-}{n}〈001〉=({-}11.8±{}0.4)×{}{10}^{{-}7}[(+34±{}3)×{}{10}^{{-}7}]$ for ${λ}=156.10nm.$ The magnitudes of these values have serious consequences for the use of crystalline optics in precision optical systems in the ultraviolet, including 193 nm and 157 nm optical lithography.